EP0514839A2 - Circuit for measuring capacity - Google Patents

Circuit for measuring capacity Download PDF

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Publication number
EP0514839A2
EP0514839A2 EP92108440A EP92108440A EP0514839A2 EP 0514839 A2 EP0514839 A2 EP 0514839A2 EP 92108440 A EP92108440 A EP 92108440A EP 92108440 A EP92108440 A EP 92108440A EP 0514839 A2 EP0514839 A2 EP 0514839A2
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EP
European Patent Office
Prior art keywords
charging
voltage
capacitance
discharge
measuring circuit
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EP92108440A
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German (de)
French (fr)
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EP0514839B1 (en
EP0514839A3 (en
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Robert Kindermann
Klaus Styhler
Herbert Ziegler
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Metrawatt GmbH
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Gossen- Metrawatt GmbH
ABB Patent GmbH
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
    • G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
    • G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
    • G01R27/2605—Measuring capacitance

Definitions

  • the invention relates to a measuring circuit for measuring a capacitance according to the preamble of claim 1.
  • the capacitance Cx to be measured is charged to a previously defined reference voltage and then discharged via a discharge resistor RE, the time from the start of the discharge to the drop in the voltage across the capacitance Cx being measured to a specific value.
  • the discharge time t is usually measured with the aid of a counter which counts clock pulses during the discharge until a comparator signals that the voltage across the capacitance has reached the specified discharge voltage.
  • switches with a small volume resistance are preferred, so that the charging resistance for the capacitance and thus the charging time constant remain as small as possible.
  • a circuit arrangement that works with an electronic switch is known from DE 28 36 324 C2. By inserting the switch into the negative feedback circuit of an operational amplifier, the volume resistance of the switch is not included in the measurement and thus does not increase the charge or discharge time constant. With a low charging resistance, you can work with a fixed charging time, because even with large capacities the charging time constant remains so small that sufficient charging is always achieved.
  • a PTC thermistor in conjunction with threshold value elements, preferably two diodes.
  • the threshold value elements ensure that overvoltages are almost short-circuited in accordance with their polarity and the resulting increased current through the PTC thermistor leads to its heating and a resulting current limitation.
  • Such protective circuits have not been used for measuring capacitances because the relatively high resistance of a PTC thermistor would negate all efforts to achieve a low charging resistance and thus a low charging time constant.
  • the fuses used instead of the PTC thermistor have the disadvantage that they have to be replaced after a blow.
  • the object of the invention is to provide a measuring circuit of the type mentioned in the preamble of claim 1, which is particularly suitable for inexpensive multimeters, contains components which protect against external voltages and which are not destroyed in the event of a fault and therefore do not have to be replaced and in which a fixed loading time can be worked without measuring errors in the case of large capacities.
  • the solution according to the invention consists first of all in a radical departure from the goal of achieving a charging time constant which is as small as possible by means of ohmic resistances in the charging circuit which are as small as possible.
  • This not only enables the use of a PTC thermistor to protect against fault voltages, but also frees you from the obligation to take special measures to reduce the volume resistance of electronic switches, which is relatively high compared to mechanical switches. If you limit the permissible capacitance measurement range to corresponding maximum values, which seems particularly acceptable with inexpensive multimeters, you can work with a fixed charging time that is still within a measurement time that is usual for such measurements, e.g. B. is at most one second.
  • monitoring means are therefore provided which monitor the respective charging voltage arising at the capacitance to be measured to determine whether it reaches the predetermined end value, this end value being slightly below the voltage value in accordance with the desired measurement accuracy the constant voltage source.
  • the monitoring means act on the sequence control or on the measured value processing in such a way that false indications are avoided and capacity overruns are recognizable.
  • a particularly advantageous development of the subject matter of the invention provides that the same comparator is preferably used as the monitoring means, which is already required anyway, in order to monitor the reaching of the predetermined discharge voltage during the discharge.
  • the comparator which assumes a certain signal state after the discharge phase has ended, is only reset to its second signal state if the voltage applied to its input during the charging phase exceeds a minimum value specified as a reference voltage. In the simplest case, this minimum value can be equal to the specified discharge voltage. If the minimum value is not reached during the loading time of the capacity, the sequential control system can interpret the lack of resetting the comparator as too large a capacity and trigger a suitable signal, possibly an overflow indicator.
  • the charging voltage reaches the specified final value during the fixed charging time. It would therefore be expedient to specify this final value as a reference value to a second comparator in order to monitor whether the permissible capacity range has been exceeded. In this case, the sequential control system could interpret the lack of a signal change on the comparator as an impermissibly large capacity.
  • An alternative to this which in turn requires only one comparator, provides that its reference voltage can be switched according to the charging or discharging phase, so that a charging voltage corresponding to the end value is monitored during the charging phase and a voltage value corresponding to the discharging voltage is monitored during the discharging phase.
  • the sequential control system can use the type of comparator signal to determine whether the measured capacitance exceeds the permissible range.
  • a significantly cheaper circuit structure compared to the measuring circuit according to DE 28 36 324 C2 can be achieved in a further development of the subject of the invention in that the semiconductor switch required for switching between charging and discharging phase is not in the negative feedback circuit of an operational amplifier, but because of its contact resistance negligible compared to the PTC thermistor is inserted in the charge / discharge circuit between the capacitance to be measured or the upstream PTC thermistor and the constant voltage source. If an operational amplifier is required at all in this circuit construction, an inexpensive type can be used, which does not have to be as demanding in terms of its slew rate as in known circuits and which accordingly also manages with a lower supply current.
  • An unknown capacitance Cx to be measured is located at the input of a measuring circuit and is charged to a predetermined final value of the charging voltage by a constant voltage source 1 during the charging phase.
  • the charging voltage of a capacitance charged via a resistor approaches the value of the voltage UK2 emitted by the constant voltage source 1 asymptotically.
  • the final value of the charging voltage is therefore determined so that it is only slightly below the constant voltage UK2 used for charging in accordance with the permissible measurement tolerance.
  • the charging current flows via a first switch S1, the switching state of which is determined by a sequence control 2 and which is closed during the charging phase.
  • a PTC thermistor RK is inserted into the charging circuit, which, in conjunction with two threshold value elements D1, D2 designed as diodes, imposes an external voltage on the measuring circuit protects. If an excessive external voltage reaches the input of the measuring circuit, it is quasi short-circuited according to its polarity via one of the threshold value elements D1, D2.
  • a Zener diode is also connected in parallel for protection, which limits the voltage drop generated by the short-circuit current at the DC voltage source GK.
  • the capacitance Cx is charged during a predefined charging time, which is dimensioned such that all capacitances within the permissible measuring range are charged to at least the predefined charging voltage.
  • the sequence control 2 opens the first switch S1 and thus initiates the discharge phase.
  • the capacitance Cx is discharged via a discharge resistor RE. This can be switched over with the help of a second switch S2, if necessary, in accordance with the partial measuring range selected in order to better adapt the discharge time. If the discharge resistor RE thereby achieves a relatively low-resistance value, so that a noteworthy leakage current would occur during charging, the sequence control must ensure that the second switch S2 is controlled in push-pull with the first switch S1, i.e. remains open as long as the first switch S1 is closed.
  • the capacitance Cx is discharged until a predetermined discharge voltage which is clearly below the charging voltage but in no way goes back to zero is reached.
  • Their voltage value corresponds to one reference voltage UR1 present at a first comparator 3, so that the comparator emits a corresponding output signal as soon as the discharge voltage falls below the first reference voltage UR1.
  • the capacitance Cx is recharged. However, if its value exceeds the permissible capacitance measuring range, the capacitance Cx does not charge to the predetermined charging voltage within the fixed charging time due to the large charging time constant caused by the PTC thermistor RK. However, if the charging voltage remains below the first reference voltage UR1, the first comparator 3 cannot return to the initial state required for the discharge.
  • the sequence control is already simulated at the beginning of the discharge in that the predetermined discharge voltage was reached in virtually zero time, so that it is an extremely small capacity. In order to avoid a resulting false indication, the sequential control system is programmed so that it monitors the first comparator 3 to determine whether its output signal experiences a signal change during the charging phase. If this is not the case, an overflow signal is generated with the aid of a display unit, which makes it recognizable that the capacitance to be measured exceeds the permissible range.
  • the capacitance to be measured is just large enough that its charging voltage falls in the range between the first reference voltage UR1 and the predetermined charging voltage, a very small capacitance is initially simulated. However, since in each measuring cycle the charging of the capacitance starts at a voltage level which is caused by the respective discharge voltage is determined, the charging voltage reaches the predetermined final value of the charging voltage in one of the following measuring cycles, so that the displayed measured value also very quickly reaches the overflow.
  • the resistors connected upstream of the two comparators 3, 4 have the task of limiting the input current.
  • One to constant voltage source 1 The associated operational amplifier is connected with a downstream transistor and two resistors as a voltage amplifier which amplifies a first constant voltage UK1 supplied to it on the input side to the second constant voltage UK2.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Resistance Or Impedance (AREA)
  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

Abstract

A measuring circuit which allows a capacitance to be measured in accordance with the discharge method is to be protected against external voltages without a component like a fusible link being destroyed and having to be exchanged in the case of a fault. As protection, a PTC resistor (RK) is used in connection with threshold elements (D1, D2) which short circuit dangerous voltages and limit the short-circuit current. Since the charging time constant is increased due to the PTC resistor (RK), the permissible capacitance measuring range is restricted to a maximum value. Transgression of the maximum value is signalled with the aid of monitoring means (3, 4) of a sequence control (4) which, in turn, ensures that there are no faulty indications and the transgression of the measuring range becomes recognisable. The measuring circuit can be used in measuring instruments for measuring capacitance and in multimeters. <IMAGE>

Description

Die Erfindung betrifft eine Meßschaltung zur Messung einer Kapazität nach den Oberbegriff des Anspruchs 1.The invention relates to a measuring circuit for measuring a capacitance according to the preamble of claim 1.

Für preisgünstige Multimeter benötigt man eine möglichst einfache Meßschaltung zur Kapazitätsmessung. Eine aus DIN 41 328, Blatt 4, Juni 1974 bekannte, nach dem Entladeprinzip arbeitende Schaltungsanordnung hat sich unter anderem auch wegen ihrer guten Meßgenauigkeit sehr bewährt. Hierbei wird die zu messende Kapazität Cx auf eine zuvor festgelegte Referenzspannung aufgeladen und danach über einen Entladewiderstand RE entladen, wobei die Zeit vom Beginn der Entladung bis zum Abfall der Spannung an der Kapazität Cx auf einen bestimmten Wert gemessen wird. Die Entladung erfolgt mit der Zeitkonstante τ = RE · Cx und die Kapazität Cx = k · t ist der gemessenen Entladezeit t entsprechend der Konstante k proportional. Die Messung der Entladezeit t erfolgt in der Regel mit Hilfe eines Zählers, der während der Entladung so lange Taktimpulse zählt, bis ein Komparator signalisiert, daß die Spannung an der Kapazität die vorgegebene Entladespannung erreicht hat.For inexpensive multimeters, the simplest possible measurement circuit for capacitance measurement is required. A circuit arrangement known from DIN 41 328, sheet 4, June 1974, which works according to the discharge principle, has also proven very useful, inter alia, because of its good measuring accuracy. Here, the capacitance Cx to be measured is charged to a previously defined reference voltage and then discharged via a discharge resistor RE, the time from the start of the discharge to the drop in the voltage across the capacitance Cx being measured to a specific value. The discharge takes place with the time constant τ = RE · Cx and the capacitance Cx = k · t corresponds to the measured discharge time t Constant k proportional. The discharge time t is usually measured with the aid of a counter which counts clock pulses during the discharge until a comparator signals that the voltage across the capacitance has reached the specified discharge voltage.

Für das Umschalten zwischen Lade- und Entladephase verwendet man bevorzugt Schalter mit kleinem Durchgangswiderstand, damit der Ladewiderstand für die Kapazität und damit auch die Ladezeitkonstante möglichst klein bleiben. Eine Schaltungsanordnung, die mit einem elektronischen Schalter arbeitet, ist aus der DE 28 36 324 C2 bekannt. Durch das Einfügen des Schalters in den Gegenkopplungskreis eines Operationsverstärkers wird erreicht, daß der Durchgangswiderstand des Schalters nicht in die Messung eingeht und somit auch nicht die Lade- oder Entladezeitkonstante erhöht. Bei niedrigem Ladewiderstand kann mit einer fest vorgegebenen Ladezeit gearbeitet werden, weil selbst bei großen Kapazitäten die Ladezeitkonstante so klein bleibt, daß immer eine ausreichende Aufladung erzielt wird.For switching between charging and discharging phases, switches with a small volume resistance are preferred, so that the charging resistance for the capacitance and thus the charging time constant remain as small as possible. A circuit arrangement that works with an electronic switch is known from DE 28 36 324 C2. By inserting the switch into the negative feedback circuit of an operational amplifier, the volume resistance of the switch is not included in the measurement and thus does not increase the charge or discharge time constant. With a low charging resistance, you can work with a fixed charging time, because even with large capacities the charging time constant remains so small that sufficient charging is always achieved.

Von besonderer Bedeutung für den Gebrauchswert eines Multimeters ist dessen Sicherung gegen Fremdspannungen, die über das Meßobjekt eingeschleift werden können oder auch bei fehlerhaftem Anschluß an ein falsches Meßobjekt am Meßeingang entstehen. Es muß deshalb dafür gesorgt werden, daß auftretende Fremdspannungen nicht zu einer Zerstörung von Bauteilen des Meßkreises führen, die einen Ausfall des Multimeters bewirken oder sogar den Bedienenden gefährden können.Of particular importance for the usage value of a multimeter is its protection against external voltages, which can be looped in via the test object or also arise at the measurement input if it is incorrectly connected to a wrong test object. It must therefore be ensured that external voltages that occur do not lead to the destruction of components in the measuring circuit, which can cause the multimeter to fail or even endanger the operator.

Es ist bekannt, den Eingang von Meßgeräten durch einen Kaltleiter in Verbindung mit Schwellwertelementen, vorzugsweise zwei Dioden, zu schützen. Die Schwellwertelemente sorgen dafür, daß Überspannungen entsprechend ihrer Polarität nahezu kurzgeschlossen werden und der hierbei auftretende erhöhte Strom über den Kaltleiter zu dessen Erwärmung und einer daraus resultierenden Strombegrenzung führt. Zur Messung von Kapazitäten hat man derartige Schutzschaltungen jedoch nicht eingesetzt, weil der relativ hohe Widerstand eines Kaltleiters alle Anstrengungen zur Erreichung eines kleinen Ladewiderstandes und damit einer kleinen Ladezeitkonstante zunichte machen würde. Die anstelle des Kaltleiters verwendeten Schmelzsicherungen haben jedoch den Nachteil, daß sie nach einem Durchbrennen ausgetauscht werden müssen.It is known to protect the input of measuring devices by means of a PTC thermistor in conjunction with threshold value elements, preferably two diodes. The threshold value elements ensure that overvoltages are almost short-circuited in accordance with their polarity and the resulting increased current through the PTC thermistor leads to its heating and a resulting current limitation. However, such protective circuits have not been used for measuring capacitances because the relatively high resistance of a PTC thermistor would negate all efforts to achieve a low charging resistance and thus a low charging time constant. However, the fuses used instead of the PTC thermistor have the disadvantage that they have to be replaced after a blow.

Aufgabe der Erfindung ist es, eine Meßschaltung der im Oberbegriff des Anspruchs 1 genannten Art zu schaffen, die sich besonders für preisgünstige Multimeter eignet, gegen Fremdspannungen schützende Bauteile enthält, die im Fehlerfall nicht zerstört werden und somit auch nicht ausgetauscht werden müssen und bei der mit einer festen Ladezeit gearbeitet werden kann, ohne daß es bei großen Kapazitäten zu Meßfehlern kommt.The object of the invention is to provide a measuring circuit of the type mentioned in the preamble of claim 1, which is particularly suitable for inexpensive multimeters, contains components which protect against external voltages and which are not destroyed in the event of a fault and therefore do not have to be replaced and in which a fixed loading time can be worked without measuring errors in the case of large capacities.

Diese Aufgabe wird durch die im Anspruch 1 gekennzeichneten Merkmale gelöst. Zweckmäßige Ausgestaltungen und Weiterbildungen des Erfindungsgegenstandes sind in den Unteransprüchen genannt.This object is achieved by the features characterized in claim 1. Appropriate refinements and developments of the subject matter of the invention are mentioned in the subclaims.

Die erfindungsgemäße Lösung besteht zunächst in einer radikalen Abkehr von dem Ziel eine möglichst kleine Ladezeitkonstante durch möglichst kleine ohmsche Widerstände im Ladekreis zu erreichen. Dies ermöglicht nicht nur die Verwendung eines Kaltleiters zum Schutz vor Fehlerspannungen, sondern befreit auch von dem Zwang besondere Maßnahmen zu treffen, um den im Vergleich zu mechanischen Schaltern relativ hohen Durchgangswiderstand von elektronischen Schaltern zu reduzieren. Schränkt man den zulässigen Kapazitätsmeßbereich auf entsprechende Höchstwerte ein, was besonders bei preisgünstigen Multimetern akzeptabel erscheint, so kann mit einer festen Ladezeit gearbeitet werden, die noch innerhalb einer für derartige Messungen üblichen Meßzeit, z. B. bei maximal einer Sekunde liegt.The solution according to the invention consists first of all in a radical departure from the goal of achieving a charging time constant which is as small as possible by means of ohmic resistances in the charging circuit which are as small as possible. This not only enables the use of a PTC thermistor to protect against fault voltages, but also frees you from the obligation to take special measures to reduce the volume resistance of electronic switches, which is relatively high compared to mechanical switches. If you limit the permissible capacitance measurement range to corresponding maximum values, which seems particularly acceptable with inexpensive multimeters, you can work with a fixed charging time that is still within a measurement time that is usual for such measurements, e.g. B. is at most one second.

Entscheidend ist, daß es bei Kapazitäten, die den zulässigen Meßbereich überschreiten, nicht zu irreführenden Fehlmessungen kommt. Ohne geeignete Zusatzmaßnahmen wäre das jedoch unvermeidlich, denn eine zu große Kapazität würde innerhalb der fest vorgegebenen Ladezeit nicht auf den vorgegebenen Endwert der Ladespannung aufgeladen. Bei einer sich an die Ladezeit automatisch anschließenden Entladung der Kapazität würde der ebenfalls vorgegebene Wert der Entladespannung schneller erreicht, weil die Entladung von einer niedrigeren Spannung aus erfolgte. Die kürzere Entladezeit würde somit eine wesentlich kleinere Kapazität vortäuschen. Erfindungsgemäß sind deshalb Überwachungsmittel vorgesehen, die die jeweilige an der zu messenden Kapazität entstehende Ladespannung dahingehend überwachen, ob sie den vorgegebenen Endwert erreicht, wobei dieser Endwert entsprechend der angestrebten Meßgenauigkeit geringfügig unter dem Spannungswert der Konstantspannungsqelle liegt. Die Überwachungsmittel wirken hierbei so auf die Ablaufsteuerung bzw. auf die Meßwertverarbeitung, daß Fehlanzeigen unterbleiben und Kapazitätsüberschreitungen erkennbar sind.It is crucial that capacities that exceed the permissible measuring range do not lead to misleading incorrect measurements. Without suitable additional measures, however, this would be unavoidable, because an excessively large capacity would not be charged to the specified final value of the charging voltage within the fixed charging time. If the capacity were to be discharged automatically following the charging time, the likewise predetermined value of the discharge voltage would be reached more quickly because the discharge took place from a lower voltage. The shorter discharge time would thus simulate a much smaller capacity. According to the invention, monitoring means are therefore provided which monitor the respective charging voltage arising at the capacitance to be measured to determine whether it reaches the predetermined end value, this end value being slightly below the voltage value in accordance with the desired measurement accuracy the constant voltage source. The monitoring means act on the sequence control or on the measured value processing in such a way that false indications are avoided and capacity overruns are recognizable.

Eine besonders vorteilhafte Weiterbildung des Erfindungsgegenstandes sieht vor, daß als Überwachungsmittel vorzugsweise derselbe Komparator dient, der ohnehin bereits benötigt wird, um bei der Entladung das Erreichen der vorgegebenen Entladespannung zu überwachen. Der nach abgeschlossener Entladephase einen bestimmten Signalzustand annehmende Komparator wird nur dann in seinen zweiten Signalzustand zurückgestellt, wenn die an seinem Eingang anliegende Spannung während der Ladephase einen als Referenzspannung vorgegebenen Mindestwert überschreitet. Im einfachsten Fall kann dieser Mindestwert gleich der vorgegebenen Entladespannung sein. Wird der Mindestwert während der Ladezeit der Kapazität nicht erreicht, so kann die Ablaufsteuerung das Fehlen der Rückstellung des Komparators als eine zu große Kapazität interpretieren und ein geeignetes Signal, ggf. eine Überlaufanzeige, auslösen.A particularly advantageous development of the subject matter of the invention provides that the same comparator is preferably used as the monitoring means, which is already required anyway, in order to monitor the reaching of the predetermined discharge voltage during the discharge. The comparator, which assumes a certain signal state after the discharge phase has ended, is only reset to its second signal state if the voltage applied to its input during the charging phase exceeds a minimum value specified as a reference voltage. In the simplest case, this minimum value can be equal to the specified discharge voltage. If the minimum value is not reached during the loading time of the capacity, the sequential control system can interpret the lack of resetting the comparator as too large a capacity and trigger a suitable signal, possibly an overflow indicator.

Bei der maximal zulässigen Kapazität erreicht die Ladespannung während der festen Ladezeit genau den vorgegebenen Endwert. Es wäre deshalb zweckmäßig, diesen Endwert einem zweiten Komparator als Referenzwert vorzugeben, um ein Überschreiten des zulässigen Kapazitätsbereiches zu überwachen. In diesem Fall könnte die Ablaufsteuerung das Fehlen einer Signaländerung am Komparator als unzulässig große Kapazität interpretieren.At the maximum permissible capacity, the charging voltage reaches the specified final value during the fixed charging time. It would therefore be expedient to specify this final value as a reference value to a second comparator in order to monitor whether the permissible capacity range has been exceeded. In this case, the sequential control system could interpret the lack of a signal change on the comparator as an impermissibly large capacity.

Eine Alternative hierzu, die wiederum mit nur einem Komparator auskommt, sieht vor, daß dessen Referenzspannung entsprechend der Lade- oder Entladephase umschaltbar ist, so daß während der Ladephase eine dem Endwert entsprechende Ladespannung und während der Entladephase ein der Entladespannung entsprechender Spannungswert überwacht wird. Auch hier kann die Ablaufsteuerung aus der Art des Komparatorsignals schließen, ob die gemessene Kapazität den zulässigen Bereich überschreitet.An alternative to this, which in turn requires only one comparator, provides that its reference voltage can be switched according to the charging or discharging phase, so that a charging voltage corresponding to the end value is monitored during the charging phase and a voltage value corresponding to the discharging voltage is monitored during the discharging phase. Here too, the sequential control system can use the type of comparator signal to determine whether the measured capacitance exceeds the permissible range.

Eine gegenüber der Meßschaltung nach der DE 28 36 324 C2 wesentlich preisgünstigerer Schaltungsaufbau ist in einer Weiterbildung des Erfindungsgegenstandes dadurch erreichbar, daß der zum Umschalten zwischen Lade- und Entladephase benötigte Halbleiterschalter nicht im Gegenkopplungskreis eines Operationsverstärkers liegt, sondern wegen seines gegenüber dem Kaltleiter vernachlässigbaren Übergangswiderstandes unmittelbar in den Lade-/Entladekreis zwischen der zu messenden Kapazität bzw dem vorgeschalteten Kaltleiter und der Konstantspannungsquelle eingefügt ist. Soweit bei diesem Schaltungsaufbau überhaupt ein Operationsverstärker benötigt wird, kann ein preisgünstiger Typ verwendet werden, an den keine so hohen Anforderungen bezüglich seiner Anstiegsgeschwindigkeit gestellt werden müssen wie bei bekannten Schaltungen und der dementsprechend auch mit einem niedrigeren Versorgungsstrom auskommt.A significantly cheaper circuit structure compared to the measuring circuit according to DE 28 36 324 C2 can be achieved in a further development of the subject of the invention in that the semiconductor switch required for switching between charging and discharging phase is not in the negative feedback circuit of an operational amplifier, but because of its contact resistance negligible compared to the PTC thermistor is inserted in the charge / discharge circuit between the capacitance to be measured or the upstream PTC thermistor and the constant voltage source. If an operational amplifier is required at all in this circuit construction, an inexpensive type can be used, which does not have to be as demanding in terms of its slew rate as in known circuits and which accordingly also manages with a lower supply current.

Bei einem sich über mehrere Teilbereiche erstreckenden besonders großen Kapzitätsmeßbereich ist es zweckmäßig den Entladewiderstand so anzupassen, daß die erzeugten Entladezeiten nicht allzu extrem auseinander liegen. Es ist deshalb ein zweiter steuerbarer Halbleiterschalter vorgesehen, der durch Umschalten eine Widerstandsänderung ermöglicht. Während relativ hochohmige Entladewiderstände auch während der Ladephase angeschlossen bleiben, da sie nur einen vernachlässigbaren Strom ziehen, ist es vorteilhaft entsprechend niederohmige Entladewiderstände während der Ladephase vom Ladekreis zu trennen und den zugehörigen zweiten Halbleiterschalter im Wechsel mit dem ersten Halbleiterschalter zu schalten.In the case of a particularly large capacitance measuring range which extends over several sub-areas, it is expedient to adapt the discharge resistance in such a way that the discharge times generated are not too far apart. It is therefore a second controllable semiconductor switch provided that enables a change in resistance by switching. While relatively high-resistance discharge resistors remain connected during the charging phase, since they only draw a negligible current, it is advantageous to separate correspondingly low-resistance discharge resistors from the charging circuit during the charging phase and to switch the associated second semiconductor switch alternately with the first semiconductor switch.

Ein Ausführungsbeispiel der Erfindung ist in der Zeichnung dargestellt und wird im folgenden näher beschrieben.An embodiment of the invention is shown in the drawing and will be described in more detail below.

Eine zu messende unbekannte Kapazität Cx liegt am Eingang einer Meßschaltung und wird während der Ladephase durch eine Konstantspannungsquelle 1 auf einen vorgegebenen Endwert der Ladespannnung aufgeladen. Bekanntlich nähert sich die Ladespannung einer über einen Widerstand aufgeladenen Kapazität asymptotisch dem Wert der von der Konstantspannungsquelle 1 abgegebenen Spannung UK2. Der Endwert der Ladespannung wird deshalb so festgelegt, daß er entsprechend der zulässigen Meßtoleranz nur geringfügig unterhalb der zur Ladung dienenden Konstantspannung UK2 liegt.An unknown capacitance Cx to be measured is located at the input of a measuring circuit and is charged to a predetermined final value of the charging voltage by a constant voltage source 1 during the charging phase. As is known, the charging voltage of a capacitance charged via a resistor approaches the value of the voltage UK2 emitted by the constant voltage source 1 asymptotically. The final value of the charging voltage is therefore determined so that it is only slightly below the constant voltage UK2 used for charging in accordance with the permissible measurement tolerance.

Ausgehend von der Konstantspannungsquelle 1 fließt der Ladestrom über einen ersten Schalter S1, dessen Schaltzustand durch eine Ablaufsteuerung 2 bestimmt wird und der während der Ladephase geschlossen ist. Zwischen den ersten Schalter S1 und die Kapazität Cx ist ein Kaltleiter RK in den Ladestromkreis eingefügt, der in Verbindung mit zwei als Dioden ausgeführten Schwellwertelementen D1, D2 die Meßschaltung vor sie gefährdenden Fremdspannungen schützt. Gelangt eine zu hohe Fremdspannung an den Eingang der Meßschaltung, so wird diese entsprechend ihrer Polarität über eines der Schwellwertelemente D1, D2 quasi kurzgeschlossen. Da der Kurzschlußstrom positiver Fremdspannungen über eine zur Stromversorgung der Konstantspannungsquelle 1 dienende Gleichspannungsquelle GK fließt, ist zum Schutz noch eine Zenerdiode parallel geschaltet, die den vom Kurzschlußstrom an der Gleichspannungsquelle GK erzeugten Spannungsabfall begrenzt.Starting from the constant voltage source 1, the charging current flows via a first switch S1, the switching state of which is determined by a sequence control 2 and which is closed during the charging phase. Between the first switch S1 and the capacitance Cx, a PTC thermistor RK is inserted into the charging circuit, which, in conjunction with two threshold value elements D1, D2 designed as diodes, imposes an external voltage on the measuring circuit protects. If an excessive external voltage reaches the input of the measuring circuit, it is quasi short-circuited according to its polarity via one of the threshold value elements D1, D2. Since the short-circuit current of positive external voltages flows via a DC voltage source GK serving to supply the constant voltage source 1, a Zener diode is also connected in parallel for protection, which limits the voltage drop generated by the short-circuit current at the DC voltage source GK.

Die Ladung der Kapazität Cx erfolgt während einer fest vorgegebenen Ladezeit, die so bemessen ist, daß alle innerhalb des zulässigen Meßbereiches liegenden Kapazitäten mindestens auf die vorgegebene Ladespannung aufgeladen werden. Im Anschluß an die Ladezeit öffnet die Ablaufsteuerung 2 den ersten Schalter S1 und leitet damit die Entladephase ein. Die Entladung der Kapazität Cx erfolgt über einen Entladewiderstand RE. Dieser kann entsprechend dem jeweils gewählten Teilmeßbereich, zur besseren Anpassung der Entladezeit, mit Hilfe eines zweiten Schalters S2 bei Bedarf umgeschaltet werden. Sofern der Entladewiderstand RE hierdurch einen relativ niederohmigen Wert erreicht, so daß während der Ladung ein nennenswerter Leckstrom entstehen würde, muß die Ablaufsteuerung dafür sorgen, daß der zweite Schalter S2 im Gegentakt mit dem ersten Schalter S1 gesteuert wird, also solange geöffnet bleibt, wie der erste Schalter S1 geschlossen ist.The capacitance Cx is charged during a predefined charging time, which is dimensioned such that all capacitances within the permissible measuring range are charged to at least the predefined charging voltage. Following the charging time, the sequence control 2 opens the first switch S1 and thus initiates the discharge phase. The capacitance Cx is discharged via a discharge resistor RE. This can be switched over with the help of a second switch S2, if necessary, in accordance with the partial measuring range selected in order to better adapt the discharge time. If the discharge resistor RE thereby achieves a relatively low-resistance value, so that a noteworthy leakage current would occur during charging, the sequence control must ensure that the second switch S2 is controlled in push-pull with the first switch S1, i.e. remains open as long as the first switch S1 is closed.

Die Entladung der Kapazität Cx erfolgt solange, bis eine vorgegebene, deutlich unter der Ladespannung liegende, aber keinesfalls bis auf Null zurückgehende Entladespannung erreicht ist. Deren Spannungswert entspricht einer an einem ersten Komparator 3 anliegenden Referenzspannung UR1, so daß der Komparator ein entsprechendes Ausgangssignal abgibt, sobald die Entladespannung die erste Referenzspannung UR1 unterschreitet.The capacitance Cx is discharged until a predetermined discharge voltage which is clearly below the charging voltage but in no way goes back to zero is reached. Their voltage value corresponds to one reference voltage UR1 present at a first comparator 3, so that the comparator emits a corresponding output signal as soon as the discharge voltage falls below the first reference voltage UR1.

Nach Abschluß der Entladephase wird die Kapazität Cx erneut geladen. Falls jedoch ihr Wert den zulässigen Kapazitätsmeßbereich überschreitet, lädt sich die Kapazität Cx aufgrund der durch den Kaltleiter RK bewirkten großen Ladezeitkonstante, innerhalb der festen Ladezeit nicht auf die vorgegebene Ladespannung auf. Bleibt aber die Ladespannung unterhalb der ersten Referenzspannung UR1, so kann der erste Komparator 3 nicht in den für die Entladung benötigten Ausgangszustand zurückkehren. Der Ablaufsteuerung wird dadurch schon zu Beginn der Entladung vorgetäuscht, daß die vorgegebene Entladespannung quasi in der Zeit Null erreicht wurde, es sich somit um eine extrem kleine Kapazität handelt. Um eine daraus resultierende Fehlanzeige zu vermeiden, ist die Ablaufsteuerung so programmiert, daß sie den ersten Komparator 3 dahingehend überwacht, ob sein Ausgangssignal während der Ladephase einen Signalwechsel erfährt. Sollte das nicht der Fall sein, wird mit Hilfe einer Anzeigeeinheit ein Überlaufsignal erzeugt, das erkennbar macht, daß die zu messende Kapazität den zulässigen Bereich überschreitet.After the discharge phase has been completed, the capacitance Cx is recharged. However, if its value exceeds the permissible capacitance measuring range, the capacitance Cx does not charge to the predetermined charging voltage within the fixed charging time due to the large charging time constant caused by the PTC thermistor RK. However, if the charging voltage remains below the first reference voltage UR1, the first comparator 3 cannot return to the initial state required for the discharge. The sequence control is already simulated at the beginning of the discharge in that the predetermined discharge voltage was reached in virtually zero time, so that it is an extremely small capacity. In order to avoid a resulting false indication, the sequential control system is programmed so that it monitors the first comparator 3 to determine whether its output signal experiences a signal change during the charging phase. If this is not the case, an overflow signal is generated with the aid of a display unit, which makes it recognizable that the capacitance to be measured exceeds the permissible range.

Ist die zu messende Kapazität gerade so groß, daß ihre Ladespannung in den Bereich zwischen der ersten Referenzspannung UR1 und der vorgegebenen Ladespannung fällt, so wird zunächst eine sehr kleine Kapazität vorgetäuscht. Da jedoch in jedem Meßzyklus das Laden der Kapazität bei einem Spannungsniveau beginnt, das durch die jeweilige Entladespannung bestimmt wird, erreicht die Ladespannung in einem der folgenden Meßzyklen den vorgegebenen Endwert der Ladespannung, so daß der angezeigte Meßwert ebenfalls sehr rasch den Überlauf erreicht.If the capacitance to be measured is just large enough that its charging voltage falls in the range between the first reference voltage UR1 and the predetermined charging voltage, a very small capacitance is initially simulated. However, since in each measuring cycle the charging of the capacitance starts at a voltage level which is caused by the respective discharge voltage is determined, the charging voltage reaches the predetermined final value of the charging voltage in one of the following measuring cycles, so that the displayed measured value also very quickly reaches the overflow.

Als Alternative zu dem gleitenden Überlauf, wie er bei bestimmten Kapazitäten auftritt, wenn zu große Kapazitäten allein durch den, mit nur einer ersten Referenzspannung UR1 beaufschlagten, ersten Komparator 3 erfaßt werden sollen, kann man auch, wie gestrichelt dargestellt, mit einer zweiten Referenzspannung UR2 arbeiten. Mit Hilfe der Ablaufsteuerung müßte ein dritter Schalter S3 jeweils so gesteuert werden, daß während der Ladephase eine zweite Referenzspannung UR2 und während der Entladephase die erste Referenzspannung UR1 eingeschaltet wird. Die zweite Referenzspannung UR2 müßte in diesem Fall der vorgegebenen Ladespannung entsprechen. Letztlich erfolgt auch hier die Signalisierung eines Überlaufs in der zuvor beschriebenen Weise.As an alternative to the sliding overflow, as occurs with certain capacitances, if excessive capacitances are to be detected solely by the first comparator 3, which is only loaded with a first reference voltage UR1, it is also possible, as shown in dashed lines, with a second reference voltage UR2 work. With the aid of the sequence control, a third switch S3 would have to be controlled such that a second reference voltage UR2 is switched on during the charging phase and the first reference voltage UR1 is switched on during the discharging phase. In this case, the second reference voltage UR2 should correspond to the specified charging voltage. Ultimately, an overflow is also signaled here in the manner described above.

Schließlich ergibt sich als weitere Alternative die Möglichkeit, anstelle der zweiten Referenzspannung UR2 einen zweiten Komparator 4 mit einer dritten Referenzspannung UR3 zu verwenden. Die der vorgegebenen Ladespannung entsprechende dritte Referenzspannung UR3 muß während der Ladephase von der Ladespannung erreicht werden, damit die Ablaufsteuerung 2 bzw eine zugehörige Meßwertverarbeitungsschaltung kein Überlaufsignal erzeugt.Finally, as a further alternative, there is the possibility of using a second comparator 4 with a third reference voltage UR3 instead of the second reference voltage UR2. The third reference voltage UR3 corresponding to the predetermined charging voltage must be reached by the charging voltage during the charging phase, so that the sequence control 2 or an associated measured value processing circuit does not generate an overflow signal.

Die den beiden Komparatoren 3, 4 eingangsseitig vorgeschalteten Widerstände haben die Aufgabe den Eingangsstrom zu begrenzen. Ein zur Konstantspannungsquelle 1 gehöriger Operationsverstärker ist mit einem nachgeschalteten Transistor und zwei Widerständen als Spannungsverstärker geschaltet der eine ihm eingangsseitig zugeführte erste Konstantspannung UK1 zur zweiten Konstantspannung UK2 verstärkt.The resistors connected upstream of the two comparators 3, 4 have the task of limiting the input current. One to constant voltage source 1 The associated operational amplifier is connected with a downstream transistor and two resistors as a voltage amplifier which amplifies a first constant voltage UK1 supplied to it on the input side to the second constant voltage UK2.

Claims (9)

Meßschaltung zur Messung einer Kapazität nach der Entlademethode, mit einer Konstantspannungsquelle (1) zum Aufladen der Kapazität (Cx), einem Entladewiderstand (RE) mit bekanntem Widerstandswert, über den eine Entladung der Kapazität (Cx) erfolgt und einer Ablaufsteuerung (2), die durch Steuerung mindestens eines entsprechend angeordneten Schalters (S1) die Lade- und Entladephase bestimmt, sowie mit einem, vorzugsweise der Ablaufsteuerung (2) zugeordneten Zähler, der während der Entladung solange Taktimpulse zählt, bis ein Komparator (3) signalisiert, daß die Entladespannung an der Kapazität (Cx) eine vorgegebene erste Referenzspannung (UR1) unterschritten hat und damit die Entladezeit beendet ist, dadurch gekennzeichnet, daß zum Schutz gegen Fehlerspannungen am Eingang der Meßschaltung in Reihe zur Kapazität (Cx) ein vom Ladestrom durchflossener Kaltleiter (RK) liegt und die sich durch den Kaltleiter (RK) erhöhende Zeitkonstante für die Aufladung durch eine Beschränkung des zulässigen Kapazitätsbereiches auf einen Höchstwert so begrenzt ist, daß die Ladezeit der Kapazität (Cx) im Rahmen üblicher Meßzeiten bleibt, und daß Überwachungsmittel (3, 4) vorgesehen sind, die so auf die Ablaufsteuerung (2) und oder eine zugehörige Meßwertverarbeitung einwirken, daß Fehlanzeigen unterbleiben und erkennbar wird, wenn innerhalb der fest vorgegebenen Ladezeit die Ladespannung einen vorgegebenen Spannungswert nicht erreicht, weil die Kapazität (Cx) den Höchstwert des zulässigen Kapazitätsbereiches überschreitet.Measuring circuit for measuring a capacitance according to the discharge method, with a constant voltage source (1) for charging the capacitance (Cx), a discharge resistor (RE) with a known resistance value, via which the capacitance is discharged (Cx) and a sequence control (2) which the charge and discharge phase is determined by controlling at least one correspondingly arranged switch (S1), and with a counter, preferably assigned to the sequence control (2), which counts clock pulses during the discharge until a comparator (3) signals that the discharge voltage is on the capacitance (Cx) has fallen below a predetermined first reference voltage (UR1) and thus the discharge time has ended, characterized in that a thermistor (RK) through which the charging current flows is connected in series with the capacitance (Cx) to protect against fault voltages and the time constant for charging due to the PTC thermistor (RK) due to a restriction The permissible capacity range is limited to a maximum value such that the loading time of the capacity (Cx) remains within the usual measurement times, and that monitoring means (3, 4) are provided which act on the sequence control (2) and or an associated measurement value processing That false indications are omitted and become recognizable if the charging voltage does not reach a specified voltage value within the fixed charging time because the capacity (Cx) exceeds the maximum value of the permissible capacity range. Meßschaltung nach Anspruch 1, dadurch gekennzeichnet, daß als Überwachungsmittel mindestens ein Komparator (3, 4) dient, der das Erreichen eines vorgegebenen Mindestwertes der Ladespannung an der Kapazität (Cx) erfaßt und ein entsprechendes Ausgangssignal der Ablaufsteuerung (2) zuführt, die ihrerseits so programmiert ist, daß sie am Ausgangssignal des entsprechenden Komparators (3,4) erkennt, ob während der Ladezeit der vorgegebene Mindestwert der Ladespannung überschritten wurde.Measuring circuit according to claim 1, characterized in that at least one comparator (3, 4) serves as monitoring means, which detects the reaching of a predetermined minimum value of the charging voltage at the capacitance (Cx) and supplies a corresponding output signal to the sequence control (2), which in turn so It is programmed that it detects from the output signal of the corresponding comparator (3, 4) whether the specified minimum value of the charging voltage has been exceeded during the charging time. Meßschaltung nach Anspruch 2, dadurch gekennzeichnet, daß als Überwachungsmittel derselbe Komparator (3) dient, der das Erreichen der vorgegebenen Entladespannung mit Hilfe einer ersten Referenzspannung (UR1) überwacht, wobei dieser während der Ladung der Kapazität (Cx) das Erreichen einer der ersten Referenzspannung (UR1) entsprechenden Ladespannung erfaßt und ein entsprechendes Signal der Ablaufsteuerung (2) zuführt.Measuring circuit according to claim 2, characterized in that the same comparator (3) serves as the monitoring means, which monitors the reaching of the predetermined discharge voltage with the aid of a first reference voltage (UR1), this during the charging of the capacitance (Cx) reaching one of the first reference voltage (UR1) corresponding charging voltage is detected and a corresponding signal is fed to the sequence control (2). Meßschaltung nach Anspruch 2, dadurch gekennzeichnet, daß als Überwachungsmittel derselbe Komparator (3) dient, der das Erreichen der vorgegebenen Entladespannung mit Hilfe einer ersten Referenzspannung (UR1) überwacht, und daß zwischen dieser und einer zweiten Referenzspannung (UR2), die der vorgegebenen Ladespannung entspricht, so umgeschaltet wird, daß der Komparator (3) während der Ladung der Kapazität (Cx) das Erreichen einer der zweiten Referenzspannung (UR2) entsprechenden Ladespannung erfaßt und ein entsprechendes Signal der Ablaufsteuerung (2) zuführt.Measuring circuit according to claim 2, characterized in that the same comparator (3) serves as monitoring means, which monitors the reaching of the predetermined discharge voltage with the aid of a first reference voltage (UR1), and that between this and a second reference voltage (UR2) that of the predetermined charging voltage corresponds, is switched so that the comparator (3) detects during the charging of the capacitance (Cx) the reaching of a charging voltage corresponding to the second reference voltage (UR2) and supplies a corresponding signal to the sequence control (2). Meßschaltung nach Anspruch 2, dadurch gekennzeichnet, daß ein zweiter Komparator (4) vorgesehen ist und dieser mit einem Eingang an einer dritten Referenzspannung (UR3) liegt, die der vorgegebenen Ladespannung entspricht, und daß der zweite Komparator (3) während der Ladung der Kapazität (Cx) das Erreichen einer der zweiten Referenzspannung (UR2) entsprechenden Ladespannung erfaßt und ein entsprechendes Signal der Ablaufsteuerung (2) zuführt.Measuring circuit according to claim 2, characterized in that a second comparator (4) is provided and this is connected with an input to a third reference voltage (UR3) which corresponds to the predetermined charging voltage, and in that the second comparator (3) during the charging of the capacitance (Cx) detects when a charging voltage corresponding to the second reference voltage (UR2) is reached and supplies a corresponding signal to the sequence control (2). Meßschaltung nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß der steuerbare erste Schalter (S1) vorzugsweise ein Halbleiterschalter ist, und zwischen der Konstantspannungsquelle (1) und dem Kaltleiter (RK) angeordnet ist und beim Öffnen des ersten Schalters (S1) die Ladung unterbrochen wird und eine Entladung über den Entladewiderstand (RE) beginnt.Measuring circuit according to one of the preceding claims, characterized in that the controllable first switch (S1) is preferably a semiconductor switch and is arranged between the constant voltage source (1) and the PTC thermistor (RK) and the charge is interrupted when the first switch (S1) is opened and a discharge via the discharge resistor (RE) begins. Meßschaltung nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß ein steuerbarer zweiter Schalter (S2) vorgesehen ist, der durch Umschalten eine Widerstandsänderung des Entladewiderstandes (RE) ermöglicht und diese vorzugsweise zur Reduzierung des Widerstandswertes vorgesehene Umschaltung bei entsprechend kleinen Widerstandswerten des Entladewiderstandes (RE) im Wechsel mit dem ersten Schalter (S1) erfolgt.Measuring circuit according to one of the preceding claims, characterized in that a controllable second switch (S2) is provided which enables a change in resistance of the discharge resistor (RE) by switching, and this switchover which is preferably provided to reduce the resistance value with correspondingly small resistance values of the discharge resistor (RE) alternating with the first switch (S1). Meßschaltung nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß ausgehend vom Eingang der Meßschaltung hinter dem Kaltleiter (RK) zwei Schwellwertelemente (D1,D2), vorzugsweise Dioden, so angeordnet sind, daß Fremdspannungen einer Polarität unmittelbar zum zweiten Pol des Eingangs der Meßschaltung und Fremdspannungen der anderen Polarität über eine zur Versorgung der Konstantspannungsquelle (1) dienende Gleichspannungsquelle (UG) zum zweiten Pol des Meßschaltungseingangs kurzgeschlossen sind.Measuring circuit according to one of the preceding claims, characterized in that, starting from the input of the measuring circuit behind the PTC thermistor (RK), two threshold value elements (D1, D2), preferably diodes, are arranged such that external voltages of one polarity are directly connected to the second pole of the input of the measuring circuit and External voltages of the other polarity are short-circuited to the second pole of the measuring circuit input via a DC voltage source (UG) serving to supply the constant voltage source (1). Meßschaltung nach Anspruch 8, dadurch gekennzeichnet, daß parallel zur Gleichspanungsquelle (UG) eine Zenerdiode (Z) angeordnet ist.Measuring circuit according to claim 8, characterized in that a zener diode (Z) is arranged parallel to the direct voltage source (UG).
EP92108440A 1991-05-24 1992-05-19 Circuit for measuring capacity Expired - Lifetime EP0514839B1 (en)

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DE4116961A DE4116961A1 (en) 1991-05-24 1991-05-24 MEASURING CIRCUIT FOR MEASURING CAPACITY

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US5329239A (en) 1994-07-12
ATE174434T1 (en) 1998-12-15
DE59209584D1 (en) 1999-01-21
EP0514839B1 (en) 1998-12-09
DE4116961A1 (en) 1992-11-26
EP0514839A3 (en) 1995-08-30

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